1. Field of the Invention
The present invention relates to a rubber composition for a tire, and more particularly, a rubber composition suitable for a tire side wall and a rubber composition suitable for a tire tread.
2. Description of the Related Art
One role of a side wall part of a pneumatic tire said to be protection of a carcass part and relaxation of stress or strain of a tire tread part during driving, and especially, the role of a side wall part of a radial tire is more important than that of a bias tire. Properties required for the side wall part for the protection of a carcass part, include weather resistance, scratch resistance during driving on a bad road and adhesion to a carcass part. As a property of the side wall required for the relaxation of stress and strain of a tread part, bending resistance is mentioned. Namely, a pneumatic tire is required to have various properties as described above since it is used under various circumstances and conditions. Of the above-described properties, the scratch resistance, bending resistance and weather resistance are highly required in particular, since maintenance of roads has improved and tire life has been further increased with spreading popularity of radial tires.
On the other hand, there are problems described below from the viewpoint of appearance. Namely, since a diene rubber used in a tire side wall has insufficient weather resistance, an amine-based antioxidant, paraffin wax and the like have been added. However, a diene rubber compounded with an amine-based antioxidant or paraffin wax has problems that the amine-based antioxidant or paraffin wax bleeds out of the surface with a lapse of time and the surface discolors to red to lower commercial value, and the amine-based antioxidant or paraffin wax is diffused during driving and ozone cracks are formed on the surface of a tire, and the like. For a formulation with which an antioxidant and the like are not compounded, it is known to use of EPT having high ozone resistance. However, a tire side wall comprising this EPT and a diene rubber has to further be improved in strength properties.
To solve the above-described problems, a rubber composition comprising natural rubber and EPDM using peroxide vulcanization has also been developed. However, such a peroxide vulcanization provides poor crack growth resistance required as property of a tire side wall, and does not provide excellent bending resistance, and therefore, it should be further improved.
Therefore, there is still a need for a rubber material for a tire side wall, which is excellent in strength property and bending resistance and provides low fuel consumption.
Further, styrene/butadiene copolymer rubber is conventionally used as rubber for tire tread of an automobile (rubber for a tire tread part). However, a tire tread comprising as a rubber component solely styrene/butadiene copolymer rubber has problems such as rolling resistance is high and abrasion resistance is poor since impact resilience at from 50 to 70.degree. C. is low, though braking ability on wet road surface (wet skid) is relatively excellent.
Therefore, as a rubber material for a tire tread of an automobile, a blend of styrene/butadiene copolymer rubber with natural rubber is generally used. However, there is desired a tire having excellent balance from viewpoints of safety and energy conservation, namely having high braking ability even on wet road surface, in addition to lower fuel consumption and abrasion resistance. The conventional blend of styrene/butadiene copolymer rubber with natural rubber can not satisfy these abilities.
As a rubber composition which can improve abrasion resistance of a tire and braking ability of an automobile under wet conditions and can lower rolling resistance, raw material rubber for a tire tread, comprising a blend of polybutadiene and halogen-containing polyisobutyrene/isoprene rubber is described in Japanese Patent Laid-Open Application (JP-A) No. 56-93738. However, even when the above-described blend is used, abrasion resistance, braking ability and rolling resistance are not satisfactory.
Therefore, a rubber material for a tire tread is required, which has excellent abrasion resistance equivalent to or more than that of natural rubber and is excellent in strength property, further, has more excellent braking property than that of natural rubber, and has low rolling resistance and can improve fuel consumption.